I just had the heat exchanger replaced on my seven year old unit (75k BTU). Per the recomendations on this site, I had the repair guy check my returns and he stated they are sized correctly. He also told me to continue using the pleated filters (Merv 5) to keep everything clean. Based on that, is there anything else I can do to prevent another premature failure? The exchanger had rusted holes the size of a pencil eraser in it. The repair man said it's just the nature of the equipment and the outside location. Doesn't leave me feeling particularly confident. Just curious... any thoughts are appreciated.